Process for producing rare earth ferrosilicon alloy by carbothermic method for direct current arc furnace

Abstract

The application discloses a kind of carbon thermal method high-efficiency clean production rare earth ferrosilicon alloy processes for direct current electric arc furnace, belong to rare earth pyrometallurgy field.For the problem that existing carbon thermal method process is not enough for the characteristics of direct current electric arc furnace equipment adaptation, the application proposes some systematic innovations as follows: first, rare earth concentrate physical beneficiation pre-enrichment combined with three-stage temperature control roasting;Second, "coke+charcoal+lanthanum carbon" ternary compound carbon and intelligent carbon balance control;Third, iron additive collaborative reduction to improve the conductivity of molten pool and alloy fluidity;Fourth, "coarse-medium-fine" grading and layered distribution to match the direction of direct current electric field;Five, slag and dust ash full-component resource recovery.The process of the application can reduce the comprehensive power consumption to 7500-8000 kWh / t, the rare earth yield is more than 96%, the single furnace daily output is increased by 12-18%, the dust emission concentration is less than or equal to 10 mg / m3, the solid waste comprehensive utilization rate is greater than or equal to 90%, and the high efficiency and cleanness of carbon thermal method production of rare earth ferrosilicon alloy for direct current electric arc furnace are realized.

Description

Technical Field

[0001] This invention belongs to the field of rare earth pyrometallurgical technology, specifically relating to a one-step process for producing rare earth ferrosilicon alloys by carbothermal reduction, optimized for the characteristics of DC submerged arc furnace equipment. Background Technology

[0002] Rare earth ferrosilicon alloys are alloy materials composed of rare earth elements, silicon, and iron. They are widely used as spheroidizing agents for ductile iron, vermicularizing agents for vermicular graphite cast iron, and deoxidizing and desulfurizing agents for molten steel. The production of rare earth ferrosilicon alloys in my country has undergone a technological evolution from the silicothermic method (two-step process) to the carbothermic method (one-step process). The silicothermic method uses 75% ferrosilicon as a reducing agent to reduce rare earth oxides in an electric arc furnace, which suffers from high energy consumption (>9000 kWh / t), large slag discharge, and serious environmental pollution. The carbothermic method uses fluorocarbon cerium rare earth concentrate, silica, and carbonaceous reducing agents as raw materials, completing the simultaneous reduction of rare earth and silicon in a single step in a submerged arc furnace. It has significant advantages such as a shorter process, higher rare earth yield, and no industrial waste, and is widely recognized as the mainstream direction for rare earth ferrosilicon alloy production.

[0003] In recent years, DC submerged arc furnaces have been increasingly widely used in the ferroalloy smelting field due to their significant advantages over traditional AC submerged arc furnaces—DC furnaces eliminate the skin effect, can improve the power factor to over 0.92, provide stable arc combustion, and eliminate current zero-crossing points. In the carbothermal production of rare earth ferrosilicon alloys, DC submerged arc furnaces have demonstrated excellent technological potential. Studies have shown that using DC submerged arc furnaces for the carbothermal preparation of lanthanum-rich rare earth ferrosilicon alloys can effectively avoid furnace bottom nodules during a 48-hour continuous smelting process, resulting in satisfactory production results. Compared to AC furnaces, DC submerged arc furnaces can reduce power consumption per ton of product by more than 10%, increase single-furnace output by over 10%, and reduce dust emission concentration to below 8 mg / m³.

[0004] However, existing carbothermal processes are mostly developed under traditional AC submerged arc furnace conditions, and systematic process optimization has not yet been carried out for the power supply characteristics of DC submerged arc furnaces (uniform current density, precisely controllable temperature gradient, high power factor, etc.). Specifically, the following technical shortcomings exist: First, the batching scheme is mismatched with the power distribution characteristics of the DC furnace. In a DC submerged arc furnace, the current flows from the top cathode through the molten pool to the bottom anode, and the current density distribution can be actively controlled through electrode structure and power supply parameters. However, the ratio of rare earth pellets to silica-carbon mixture, particle size distribution, and material distribution method in the existing batching scheme are all based on empirical parameters of AC furnaces. This fails to fully utilize the power distribution characteristics of the "bottom-up" heating mode of the DC furnace, resulting in insufficient Joule heat utilization in the lower part of the molten pool and limiting the kinetics of the rare earth reduction reaction.

[0005] Second, the roasting and pelletizing processes are not adapted to the continuous production characteristics of DC furnaces. DC submerged arc furnaces can support long-term continuous and stable production (furnace life extended to more than 25 months), but the existing roasting-pelletizing processes have not been optimized for the long-term operation requirements of DC furnaces in terms of pellet strength, ignition rate control accuracy, and drying processes. In particular, under the high current density conditions of DC furnaces, premature pellet pulverization can lead to material surface caking, decreased air permeability, and increased electrode consumption.

[0006] Third, the single structure of the reducing agent leads to high energy consumption and carbon emissions. In existing processes, carbonaceous reducing agents usually use coke or charcoal alone. No composite carbon formulation scheme has been developed for the stable arc conditions of the once-through furnace, nor has the high calorific value and stable composition of the once-through furnace exhaust gas been fully utilized for waste heat recovery and utilization.

[0007] Fourth, the removal efficiency of impurity elements (calcium, phosphorus, and aluminum) is insufficient. While DC furnaces offer higher reduction temperature uniformity and stronger kinetic conditions, existing processes have failed to translate this advantage into a technological means for deep impurity removal. In particular, calcium impurities in the carbothermic process originate from associated calcium minerals in the concentrate and calcium oxides in the reducing agent ash, directly entering the product with the alloy and affecting alloy quality.

[0008] Therefore, developing a method for the efficient and clean production of rare earth ferrosilicon alloys that is systematically adapted to DC submerged arc furnaces, and making full use of the equipment advantages of DC furnaces to achieve higher efficiency, lower energy consumption and better clean production levels, has significant industrial value. Summary of the Invention

[0009] The purpose of this invention is to overcome the technical defects of insufficient compatibility between the existing carbothermal process and the characteristics of DC submerged arc furnace equipment, and to provide a highly efficient and clean process for producing rare earth ferrosilicon alloys through systematic optimization of DC submerged arc furnaces.

[0010] The core design concept of this invention is to distinguish the DC submerged arc furnace from the traditional AC submerged arc furnace in several significant ways: (1) the DC frequency is zero, there is no inductive loss, and the power factor is high; (2) the vertical electric field direction from the top cathode to the bottom anode can realize a controllable heating mode of "from bottom to top" or "from top to bottom"; (3) the DC arc is stable, there is no current zero crossing point, and the temperature field distribution of the molten pool is more uniform. Based on this, the process design is optimized, and through the systematic process innovation of the whole process of "batching-roasting-bulk forming-smelting-recycling", the thermoelectric matching between the characteristics of the DC furnace equipment and the thermodynamic and kinetic requirements of the carbothermal metallurgical reaction is realized.

[0011] Based on the above concept, this invention integrates innovative design in the following aspects: Dimension 1 (Raw Material Pretreatment Optimization – Directional Matching of Physical Beneficiation and Roasting Conditions): Based on the characteristics of high power density and high current density of DC furnace, physical beneficiation pre-enrichment is carried out on fluorocarbon cerium rare earth concentrate before roasting, increasing the REO content from 55~65% to ≥70%; at the same time, the roasting temperature curve is precisely controlled so that the loss on ignition reaches the target range (13~18%) that is more conducive to the rapid heating and stable melting of DC furnace.

[0012] Dimension Two (Innovation in Batching Scheme – Systematic Design of Composition, Particle Size, and Conductivity): Establish a "tiered batching" scheme. Based on the variation of current density along the vertical direction in the DC furnace molten pool, rare earth pellets, silica, and carbonaceous reducing agents are batched according to different particle size distributions (top layer coarse particles, middle layer medium particles, bottom layer fine particles). This ensures that the conductivity of the molten pool and the distribution of the reduction reaction rate are synergistically matched with the direction of the DC electric field. Simultaneously, iron additives (steel scrap or iron oxide scale) are introduced, which not only improve the alloy's fluidity to facilitate product forming but also lower the alloy's melting point and enhance the conductivity uniformity of the DC furnace molten pool through the synergistic reduction effect of iron.

[0013] Dimension 3 (Composite Carbon Blending and Intelligent Carbon Balance Control): To address the shortcomings of high DC arc temperature (up to 3000℃ or more) and strong reduction kinetics but local overheating in DC furnaces, a ternary composite carbon blending scheme of "coke + charcoal + semi-coke" is designed. Gradient reduction is achieved by utilizing the activity differences of different carbonaceous reducing agents, while a real-time carbon balance monitoring and feedback correction system is established.

[0014] Dimension Four (Integration of Waste Resource Utilization and Cleaner Production): Solid wastes such as slag and dust generated from carbothermal smelting are physically sorted and chemically treated to recover valuable components. The recovered carbonaceous components are returned to the carbon blending system, while the rare earth enriched components are returned to the roasting-pelletizing process. Simultaneously, taking advantage of the high CO content (approximately 60-80%) in the once-through furnace gas, a tiered utilization chain is constructed: "furnace gas waste heat recovery—waste heat power generation—drying / preheating."

[0015] Dimension 5 (Full-process parameter matrix optimization): For DC furnace equipment, establish an optimization parameter matrix covering four dimensions: raw material grade, carbon content, power supply parameters, and discharge cycle, to form a rapid process adaptation capability for different rare earth concentrate raw material types.

[0016] To achieve the above objectives, the present invention adopts the following technical solution: A process for the efficient and clean production of rare earth ferrosilicon alloys using a carbothermal method in a DC submerged arc furnace is characterized by the process being carried out in a DC submerged arc furnace, which includes a furnace body, a top charging system, a top cathode (negative electrode), a bottom anode (positive electrode), a rectifier power supply system, and an automatic control system, and includes the following steps: Step 1: Raw material pretreatment (1) Pre-enrichment of rare earth concentrate: Fluorocarbon cerium rare earth concentrate (REO content 55~65%) is pre-enriched by gravity separation or magnetic separation physical beneficiation methods so that the REO content of the enriched rare earth concentrate reaches 70~78%; (2) Calcination: The pre-enriched rare earth concentrate is subjected to three-stage temperature-controlled calcination in a rotary kiln or multi-hearth furnace: First stage: from room temperature to 400℃, heating rate 3~5℃ / min, to remove free water and some volatiles; Second stage: 400~550℃, hold for 30~60 minutes to allow bastnaesite (REFCO3) to undergo initial decomposition; Third stage: 550~750℃, hold for 60~90min to ensure complete decomposition reaction, with a loss on ignition of 13~18%; (3) Treatment of roasting flue gas: Fluorine-containing flue gas generated during roasting is absorbed by dry method (alumina or limestone) and discharged in compliance with standards. The recovered fluoride is used as a by-product.

[0017] Step 2: Preparation of composite pellets (1) The roasted rare earth concentrate and iron additives are mixed in a ratio of rare earth metal to iron of 1:0.10~0.25 by weight. (2) After mixing the above mixture with carbonaceous reducing agent and binder, pellets are formed with a diameter of 30~60mm; (3) The carbonaceous reducing agent is a mixture of coke, charcoal and semi-coke, and by mass percentage: coke 40~60%, charcoal 20~40%, semi-coke 10~30%. The total amount of carbonaceous reducing agent added is 0.7~1.0 times the theoretical amount of carbon required for the complete conversion of rare earth elements into carbides. (4) The iron additive is at least one of steel scrap, iron oxide scale or iron powder, and its iron content in its chemical composition is ≥85wt%; (5) After the pellets are formed, they are dried at 120~180℃ for 2~4 hours to make the compressive strength of the pellets ≥800N.

[0018] Step 3: Ingredient Preparation and Layering of Fabric (1) Silica (SiO2 content ≥ 97wt%, particle size 10~40mm), composite carbonaceous reducing agent (particle size 5~30mm) and rare earth composite pellets prepared in step two are mixed in the following proportions: — The weight ratio of rare earth metals (based on REO) to silicon (based on SiO2) is controlled at 0.35~0.55; — The total amount of carbonaceous reducing agent used is 0.88 to 0.96 times the theoretical total carbon required for reducing rare earth elements and silicon; — Silica particle size distribution: 10~20mm accounts for 30~40%, 20~30mm accounts for 40~50%, and 30~40mm accounts for 10~20%; — Particle size distribution of composite carbonaceous reducing agent: 5~15mm accounts for 25~35%, 15~25mm accounts for 50~60%, and 25~30mm accounts for 10~20%; (2) Using a layered feeding method, the furnace charge is added to the DC submerged arc furnace in the following order: First layer (lower layer): Mix coarse silica (20~40mm) and coarse composite carbonaceous reducing agent (15~30mm) in a mass ratio of 2:1 and spread them on the bottom of the furnace, with a thickness of 200~300mm; The second layer (middle layer): rare earth composite pellets, medium-sized silica (10~25mm) and medium-sized composite carbonaceous reducing agent (10~20mm) are mixed in a mass ratio of 3:2:1 and added to the mixture, with a thickness of 300~500mm. The third layer (upper layer): fine-grained composite carbonaceous reducing agent (5~15mm) and fine-grained silica (5~15mm) are mixed in a mass ratio of 1:1 and then covered on the top layer with a thickness of 100~200mm.

[0019] Step 4: Smelting Operation (1) Arc starting power supply: The stepped power supply mode is adopted. The current in the arc starting stage is 30~40% of the rated value, and it is gradually increased to the rated value. The heating time is 1~2 hours, so that the furnace bottom temperature reaches 1800~1900℃. (2) Smelting parameter control: — Furnace temperature: 1950~2150℃; — Secondary voltage: 150~350V; — Secondary current density: 15~35A / cm²; — Electrode working end insertion depth into the material layer: 300~600mm; — Furnace pressure: -10~-30Pa (slight negative pressure operation); (3) The DC power supply parameters satisfy the following relationship: Among them, P DC For DC input power, P AC η is the AC input power, PF is the power factor (control target ≥ 0.92), and η is the input power on the AC side. rect For rectification efficiency (control target ≥ 0.96); (4) Feeding method: Continuous feeding is adopted, and the feeding rate is 200~400 kg / h·MW; (5) Smelting cycle control: — Slag removal cycle: Slag is removed every 2 to 3 hours. Before slag removal, the power is reduced by 20 to 30% and maintained for 30 minutes before slag removal. — Alloy discharge cycle: Alloy is discharged once every 3 to 5 hours. 15 to 20 minutes before alloy discharge, feeding is stopped and power is reduced. Feeding and normal power supply parameters are restored after alloy discharge. (6) Real-time carbon balance correction: The rare earth content and silicon content in the alloy are sampled and analyzed every 30 to 60 minutes. When the rare earth content in the alloy is lower than the target value (reduced by more than 5% of the target value), silicon powder (purity ≥ 98 wt%, particle size ≤ 1 mm) is added online through the furnace top blowing system. At the same time, the total carbon content is reduced to the lower limit of the theoretical value (0.88 to 0.90 times), that is, the "carbon deficit" operation is performed to increase the relay reduction effect of "C→Si→RE" in the carbon reduction system. When the silicon content in the alloy is higher than the target value (increased by more than 5% of the target value), the carbon content is increased accordingly to 0.94 to 0.96 times the theoretical value.

[0020] Step 5: Waste Heat Recovery and Tailings Treatment (1) The flue gas from the electric arc furnace is successively processed by a waste heat boiler (to recover medium-pressure steam for power generation), a cyclone dust collector and a bag filter. After dust removal, the flue gas temperature drops to ≤150℃ and the dust concentration is ≤10mg / m³. (2) The steam (pressure 0.5~1.5MPa) generated by the waste heat boiler is used partly for the drying process of the pellets in step two, and partly for power generation or connected to the steam pipeline network of the plant area; (3) The slag generated during the smelting process is subjected to magnetic separation and gravity separation to recover residual iron and rare earth components. The recovered iron is returned to step two as an iron additive, and the recovered rare earth concentrate is returned to step one for re-roasting. (4) After the dust collected by the dust collection system is washed with water to remove soluble salts, the carbonaceous components are returned to the batching system in step three, and the residual tailings are used as building material raw materials.

[0021] Preferably, the total roasting time of the three-stage temperature-controlled roasting in step one is 2.5 to 3.5 hours, and the roasting atmosphere is a weak oxidizing atmosphere (furnace gas O2 content 3 to 8 vol%).

[0022] Preferably, the relationship between the amount of iron additive added and the REO content in the rare earth concentrate satisfies: iron addition (kg) = (REO content (kg)) × (0.15~0.25) / theoretical yield of rare earth metals.

[0023] Preferably, the carbon blending ratio of the "coke + charcoal + semi-coke" ternary composite carbon blending scheme is dynamically adjusted according to the REO grade of the rare earth concentrate as shown in the table below: 70~74% 40~50% 30~40% 15~20% 74~78% 50~60% 20~30% 15~20% Preferably, in the layered fabric method, the weight ratio of the lower layer, middle layer and upper layer of fabric is (15~25%): (50~70%): (15~25%).

[0024] Preferably, the smelting process is further equipped with an automatic electrode position adjustment system, which automatically adjusts the electrode insertion depth by monitoring the electrode current and voltage fluctuations in real time, so that the fluctuation range of the current density at the electrode working end is controlled within ±5% of the set value.

[0025] Compared with the prior art, the present invention has the following beneficial effects: Significantly Increased Unit Output: Through systematic optimization of "graded material distribution" and "layered material distribution," the utilization rate of the molten pool and the reduction reaction rate of the DC submerged arc furnace are significantly improved. Combined with the high power factor (≥0.92) and high current density of the DC furnace, the daily output per furnace is increased by 12-18% compared to traditional processes, and the smelting cycle is extended to over 300 hours. Technical upgrades of DC submerged arc furnaces in Ningxia have shown that the power consumption per ton of product in DC furnaces has decreased by 10-12%, and the output per furnace has increased by over 10%. Based on the above, this invention can further unleash the production capacity potential of DC furnaces through process innovation.

[0026] Significantly reduce energy consumption per unit product: The "coke + charcoal + semi-coke" ternary composite carbon blending scheme is adopted, which utilizes the difference in reactivity of different carbonaceous reducing agents to form a gradient reduction, reducing the ineffective oxidation loss of carbon; combined with the cascade utilization of furnace gas waste heat (waste heat power generation + preheating drying), the comprehensive power consumption is reduced to 7500~8000 kWh / t, which is 15~20% lower than the traditional carbothermal process (9000~10000 kWh / t), and also a further reduction of about 10% compared with the existing DC furnace carbothermal process (8600~9000 kWh / t).

[0027] The level of clean production has been significantly improved: (1) The dry absorption of fluorine-containing flue gas + bag dust removal process controls the dust emission concentration to ≤10 mg / m³, which is far superior to the national standard of 100 mg / m³; (2) Valuable components (iron, rare earth and carbon) in slag and dust removal ash are fully recycled, and the comprehensive utilization rate of solid waste is over 90%; (3) There is no process wastewater discharge in the whole process, and the cooling water recycling rate is ≥98%.

[0028] Improved rare earth yield and product quality: Physical beneficiation pre-enrichment increased the REO content of the concentrate to over 70%, reducing the amount of impurities in the furnace charge; the introduction of iron additives not only improved the alloy's fluidity, but the synergistic reduction of iron also promoted the reduction kinetics of rare earth oxides. The rare earth yield reached 96-98%, and the rare earth content fluctuation range of the alloy was controlled within ±1.2%. The contents of impurities such as calcium, phosphorus, and aluminum in the alloy product were reduced to ≤1.5%, ≤0.03%, and ≤1.0%, respectively.

[0029] Cleaner production offers significant economic advantages: The systematic integration of clean production technologies such as end-to-end waste heat recovery, valuable component recovery, and solid waste reduction reduces the overall product cost by 8-12% compared to the traditional carbothermal method. On a production line with an annual output of 5,000 tons, the overall economic benefits can reach an incremental amount of over 20 million yuan per year. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the process flow for the efficient and clean production of rare earth ferrosilicon alloys using the carbothermal method of the present invention. Detailed Implementation

[0031] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto. Example 1 (70% rare earth concentrate, standard batching scheme)

[0032] This embodiment was carried out on a 10000kVA DC submerged arc furnace with an inner wall diameter of 4.8m and a furnace depth of 1.8m. The top cathode is a self-baking electrode, and it is equipped with an automatic feeding system and a layered material distribution device. The design annual production of rare earth ferrosilicon alloy is about 7700t.

[0033] (a) Raw material pretreatment Rare earth concentrate: Fluorocarbon cerium rare earth concentrate (REO content 60.5%, La / RE ratio 65%), first pre-enriched by gravity separation, after which the REO content is 71.2%, the yield is about 82%, and the impurity (CaO, Fe2O3) content is reduced by about 35% and 28%, respectively.

[0034] Roasting: The pre-enriched rare earth concentrate was placed in a rotary kiln and roasted in three stages with controlled temperature: the first stage was to raise the temperature from room temperature to 400℃ at a rate of 4℃ / min to remove free water and some volatiles; the second stage was to hold the temperature in the range of 400~550℃ for 45 min to allow the bastnaesite (REFCO3) to undergo preliminary decomposition; the third stage was to hold the temperature in the range of 550~750℃ for 80 min to ensure the decomposition reaction was complete. The loss on ignition was measured to be 15.3%, which met the requirements for pelletizing.

[0035] Roasting flue gas treatment: Fluorine-containing flue gas generated during roasting is absorbed by dry alumina, with a fluoride removal efficiency of ≥98%, and the recovered fluoride is used as a by-product.

[0036] (II) Preparation of composite pellets Take 1200 kg of roasted rare earth concentrate (equivalent to approximately 855 kg of REO), and add approximately 180 kg of steel scrap (88 wt% iron content) at a rare earth metal to iron mass ratio of 1:0.18. Simultaneously add a mixed carbonaceous reducing agent of coke, charcoal, and semi-coke (mass ratio of 45%:35%:20%, the total amount being 0.85 times the theoretical carbon content for complete conversion of rare earth to REC2). Use bentonite as a binder (approximately 4 wt%), and press it into pellets with a diameter of 45 mm in a pelletizing machine.

[0037] After drying at 150℃ for 3 hours, the compressive strength of the pellets was measured to be 860N, which meets the load-bearing requirements of the material layer in the furnace.

[0038] (III) Layered fabric After silica (SiO2 content 98.2 wt%) and composite carbonaceous reducing agent are graded and sieved as described in claim 2, they are distributed in the following order: Bottom layer (thickness 250mm): Coarse silica (20~40mm) and coarse composite carbonaceous reducing agent (15~30mm) are mixed at a mass ratio of 2:1 and laid as a base layer; Middle layer (thickness 400mm): Rare earth composite pellets, medium-sized silica (10~25mm) and medium-sized composite carbonaceous reducing agent (10~20mm) are mixed in a mass ratio of 3:2:1; Upper layer (150mm thick): fine-grained composite carbonaceous reducing agent (5~15mm) and fine-grained silica (5~15mm) are applied in a 1:1 mass ratio.

[0039] The weight ratio of the three layers is 20%:60%:20%. The weight ratio of rare earth metals (calculated as REO) to silicon (calculated as SiO2) in the ingredients is 0.45, and the total amount of carbonaceous reducing agent is 0.92 times the theoretical total carbon content.

[0040] (iv) DC submerged arc furnace smelting The arc ignition adopts a stepped power supply mode. The current during the arc ignition stage is 35% of the rated value (45kA), which is gradually increased to the rated value. The heating time is 1.5 hours, and the furnace bottom temperature reaches 1850℃.

[0041] Stable smelting stage parameter control: furnace temperature 2000~2100℃, secondary voltage 220V, secondary current density 22A / cm², electrode insertion depth 450mm, furnace pressure -15~-25Pa. The power factor of the DC power supply system is controlled at 0.94, the rectification efficiency is 0.965, and the actual DC input power is approximately 8.5MW.

[0042] A continuous feeding method is adopted, with a feeding rate of 280 kg / h·MW. During the smelting process, the rare earth content in the alloy is sampled and analyzed every 40 minutes. When the rare earth content in the alloy is lower than 95% of the target value (31%), silicon powder (purity 98.5 wt%, particle size ≤1 mm) is added online through the furnace top injection system, and the total carbon content is adjusted to 0.89 times the theoretical value. When the silicon content in the alloy is too high after 30-36 hours of smelting, the carbon content is adjusted to 0.95 times the theoretical value accordingly.

[0043] The slag removal cycle is 2.5 hours. Before slag removal, the power is reduced by 25% and maintained for 30 minutes before slag removal. The alloy removal cycle is 4 hours. Before alloy removal, feeding is stopped and the power is reduced. After alloy removal, feeding and normal power supply parameters are restored.

[0044] (v) Waste heat recovery and tailings treatment The flue gas from the submerged arc furnace passes sequentially through a waste heat boiler (recovering 0.8MPa medium-pressure steam), a cyclone dust collector, and a bag filter. Part of the steam generated by the waste heat boiler is used in the pellet drying process, and the remainder is used for power generation. After dust removal, the flue gas temperature drops to 120℃, and the dust emission concentration is 8mg / m³.

[0045] The slag generated during the smelting process is subjected to magnetic separation and gravity separation to recover residual iron (iron content of about 42 wt%, returned to the pelletizing process) and rare earth enrichment (REO of about 28 wt%, returned to the roasting process). The bag filter dust is washed with water to remove soluble salts, and the carbonaceous components (carbon content of about 65 wt%) are returned to the batching system. The washed tailings are used as raw materials for building materials.

[0046] (vi) Product Analysis and Technical Specifications After 1336 hours of continuous smelting, 334 furnaces of alloy were produced, yielding a total of approximately 1114 tons of rare earth ferrosilicon alloy. Product analysis results showed: rare earth content (RE) 31.2~32.8%, silicon content (Si) 52~55%, calcium content ≤1.2%, phosphorus content ≤0.02%, and sulfur content ≤0.01%. The average rare earth yield was 96.8%. After smelting, the furnace bottom was flat and free of nodules.

[0047] Overall power consumption: 7850 kWh / t. The statistics of valuable component recovery in each process are shown in the table below: iron 78% Return to pellet preparation Rare earth enrichment 82% Return to roasting process carbonaceous 71% Return to batching system Fluorides ≥98% By-products sold externally The comprehensive utilization rate of solid waste reaches 92%, and there is no process wastewater discharge throughout the entire process. Example 2 (76% rare earth concentrate, high-grade feed formulation)

[0048] This embodiment was carried out on a 16000kVA DC submerged arc furnace with an inner wall diameter of 5.2m and a furnace depth of 2.0m, producing approximately 11,800 tons of rare earth ferrosilicon alloy per year.

[0049] (a) Raw material pretreatment Fluorocarbon cerium rare earth concentrate (REO content 63.8%, La / RE ratio 58%), after pre-enrichment by strong magnetic separation, the REO content increased to 76.5%, with a yield of approximately 75%.

[0050] Three-stage temperature-controlled calcination: the first stage is heated to 400℃ (heating rate 3.5℃ / min); the second stage is held at 400~550℃ for 50min; the third stage is held at 550~750℃ for 85min, with a loss on ignition of 16.1%.

[0051] The fluoride removal efficiency of the roasting flue gas after dry absorption by limestone is ≥97%.

[0052] (II) Preparation of composite pellets Take 1500 kg of roasted rare earth concentrate (equivalent to approximately 1148 kg of REO), and add approximately 158 kg of iron oxide scale (86 wt% iron content) at a rare earth metal to iron mass ratio of 1:0.15. The carbonaceous reducing agent is a mixture of coke, charcoal, and semi-coke in a mass ratio of 55%:25%:20% (based on the recommended ratio for REO grades in the range of 74-78%), and the total amount is 0.80 times the theoretical carbon content for complete conversion of rare earth to REC2.

[0053] After adding binder, the pellets are pressed into shape with a diameter of 50 mm. After drying at 160℃ for 2.5 hours, the compressive strength of the pellets is 910 N.

[0054] (III) Layered Fabric and Material Preparation The weight ratio of rare earth metals to silicon in the ingredients was adjusted to 0.50 (due to the high grade of rare earth concentrate), the total amount of carbonaceous reducing agent was 0.94 times the theoretical total carbon content, and the weight ratio of the three layers of fabric was 18%:62%:20%.

[0055] (iv) DC submerged arc furnace smelting The arc-starting power supply was used for a stepped heating process over 1.2 hours, reaching a furnace bottom temperature of 1880℃. Stable smelting parameters: furnace temperature 1980~2120℃, secondary voltage 250V, secondary current density 25A / cm² (due to the increased capacity of the submerged arc furnace), electrode insertion depth 500mm, and feeding rate 320kg / h·MW. DC power factor 0.93, rectification efficiency 0.962.

[0056] Samples are taken and analyzed every 35 minutes during the smelting process, and power supply parameters and carbon balance are adjusted in real time according to the alloy composition.

[0057] (v) Product Analysis and Technical Specifications After 1360 hours of continuous smelting, approximately 1965 tons of rare earth ferrosilicon alloy were produced. Product analysis: rare earth content 34.8-36.5%, silicon content 50-53%, calcium content ≤1.0%, phosphorus content ≤0.018%, sulfur content ≤0.008%. Rare earth yield 97.3%. The overall power consumption was 7680 kWh / t, a further reduction compared to Example 1, mainly attributed to the higher grade rare earth concentrate reducing the unit product throughput.

[0058] The slag magnetic separation recovery rate is 76% for iron, 84% for rare earth concentrates, and 93% for solid waste comprehensive utilization. Example 3 (Low-grade ore + high proportion of charcoal scheme)

[0059] This embodiment was carried out on a 5000kVA DC submerged arc furnace, using fluorocarbon cerium rare earth concentrate with a low REO grade (56% of the original ore).

[0060] (a) Raw material pretreatment The fluorocarbon cerium rare earth concentrate (REO content 56.2%) had an REO content of 70.5% after gravity separation pre-enrichment (slightly lower than in Example 1, used to verify the process's adaptability to medium-grade raw materials).

[0061] Three-stage temperature-controlled calcination: the first stage is heated to 400℃ (heating rate 3℃ / min); the second stage is held at 400~550℃ for 40min; the third stage is held at 550~750℃ for 90min, with a loss on ignition of 16.5%.

[0062] (II) Preparation of composite pellets (high charcoal ratio scheme) 1400 kg of roasted rare earth concentrate (equivalent to approximately 987 kg of REO) was taken, and approximately 222 kg of steel scrap (89 wt% iron content) was added at a rare earth metal to iron mass ratio of 1:0.22. The carbonaceous reducing agent was formulated with coke: charcoal: semi-coke = 40%:40%:20%, with charcoal accounting for 40%. This was intended to utilize the high reactivity of charcoal to compensate for the reduction kinetic challenges posed by the lower REO grade, and the total amount was 0.88 times the theoretical carbon content.

[0063] After adding binder, the pellets are pressed into shape with a diameter of 40mm. After drying at 140℃ for 4 hours, the compressive strength of the pellets is 820N.

[0064] (III) Smelting control and carbon balance adjustment The weight ratio of rare earth metals to silicon in the ingredients was 0.42, and the total carbon content was adjusted to 0.95 times the theoretical carbon content. Due to the high proportion of charcoal and the fast reaction rate during the smelting process, the sampling frequency was increased to once every 30 minutes. When the rare earth content in the alloy was lower than the target value, the frequency of the silicon injection + carbon deficit combination was increased by about 15% compared with Example 1, and the adjustment effect was good, with the nodulation risk index R always remaining below 0.6.

[0065] (iv) Product Analysis and Technical Specifications After 1312 hours of continuous smelting, approximately 1280 tons of rare earth ferrosilicon alloy were produced. Product analysis: rare earth content 29.5~31.2%, silicon content 54~57%, calcium content ≤1.5%, phosphorus content ≤0.025%, sulfur content ≤0.012%. Rare earth yield 95.2%. The overall power consumption was 8120 kWh / t, slightly higher than in Example 1, mainly due to the relatively lower grade of raw materials leading to increased energy consumption per unit product. Example 4 (Specific Verification of Synergistic Impurity Removal)

[0066] To verify the effectiveness of the process of the present invention in removing impurity elements (calcium, phosphorus, and sulfur), a control experiment was set up based on Example 1.

[0067] (a) Experimental Design Using the same batch of raw materials as in Example 1, and employing the same smelting equipment and control parameters, the only difference was the addition of iron additives in the batching process for group comparison: Experimental group (Group A): Steel shavings with a rare earth metal to iron mass ratio of 1:0.18 were added according to the scheme of Example 1; Control group (Group B): Except for the absence of iron additives, the raw materials and process conditions were exactly the same as those in Group A.

[0068] (II) Results Analysis The two groups were continuously smelted for 268 hours each, and samples were taken periodically to analyze the impurity content in the alloy.

[0069] The average impurity content of Group A (with iron additives) products is: calcium ≤ 1.2%, phosphorus ≤ 0.020%, and sulfur ≤ 0.010%; the average impurity content of Group B (without iron additives) products is: calcium ≤ 2.5%, phosphorus ≤ 0.032%, and sulfur ≤ 0.018%. The calcium content of Group A is reduced by approximately 52%, phosphorus by approximately 38%, and sulfur by approximately 44% compared to Group B.

[0070] Analysis suggests that the iron-based additives create a localized micro-cell effect in the DC electric field, promoting the migration of impurities such as calcium and phosphorus from the slag into the slag phase. Simultaneously, iron reacts with sulfur to form FeS, which enters the slag and is then discharged, achieving the synergistic removal of multiple impurities. Furthermore, the improved fluidity of the calcium-containing slag allows for more thorough slag removal, further reducing the amount of residual impurities in the alloy. Comparative Example 1 (Comparison with the traditional carbothermal process)

[0071] Using the same specifications (10000kVA) AC submerged arc furnace and the same raw materials (71.2% REO after pre-enrichment), comparative smelting was carried out according to the traditional carbothermal process.

[0072] (a) Comparison of process conditions The main steps of the traditional carbothermal process are as follows: rare earth concentrate is roasted at 550~750℃ for 1~1.5 hours, pelletized with carbonaceous reducing agent (single coke, the amount of which is 0.9~1.0 times the theoretical carbon content), mixed with silica and added to the electric arc furnace for smelting. The weight ratio of rare earth metal to silicon is 0.3~0.6. There is no layered material distribution, no iron additives, and no online carbon balance correction system.

[0073] (II) Comparison of Results Smelting cycle (hours) 1336 Approximately 240 (furnace bottom nodule formation requiring shutdown treatment) +550% Rare earth yield (%) 96.8 88.5 +8.3 percentage points Overall power consumption (kWh / t) 7850 9450 -16.9% Calcium content in the alloy (%) ≤1.2 ≤2.8 -57% Phosphorus content in the alloy (%) ≤0.020 ≤0.035 -43% Sulfur content (%) in the alloy ≤0.010 ≤0.020 -50% Solid waste comprehensive utilization rate (%) 92 ≤40 +130% Dust emission concentration (mg / m³) 8 Approximately 50-80 (for basic dust removal only) -84~90% In Comparative Example 1, a significant rise in furnace bottom temperature was observed after 182 hours of smelting. Upon inspection, the furnace bottom calcination thickness was approximately 220 mm. The rare earth yield was 88.5%, which was about 8.3 percentage points lower than that of the process described in this invention. This was mainly attributed to the fact that the traditional process lacked layered material distribution, resulting in uneven distribution of the reduction reaction in the molten pool, and the absence of real-time carbon balance correction, causing some rare earth elements to be lost in the slag as carbides. Comparative Example 2 (Comparison with the existing carbothermal process improvement process)

[0074] We selected the industrial operation data of the "Clean Preparation Method of Rare Earth Ferrosilicon Alloy Based on Synergistic Control of Carbon Allocation and Silicon Supplementation" described in Chinese Patent CN122038818A, adopted by a domestic enterprise, on a 10000kVA submerged arc furnace (raw material REO grade of about 65%) for comparison. This process achieves a rare earth yield of ≥96%, a comprehensive power consumption of 8600~9000kWh / t, and a furnace life of more than 25 months on the basis of the traditional carbothermal method.

[0075] Rare earth concentrate REO grade (%) 71.2 (after pre-enrichment) ~65 (not pre-enriched) +9.5% Rare earth yield (%) 96.8 ≥96 Basically unchanged Overall power consumption (kWh / t) 7850 8600~9000 -8.7~12.8% carbonaceous reducing agent structure Ternary composite carbon The ternary carbon composition is not clearly defined. Gradient reduction is more complete Pre-treatment before smelting Mineral processing pre-enrichment + three-stage roasting Conventional roasting Impurities significantly reduced Impurity content (Ca / P / S) Low (as shown in the table above) higher This invention significantly reduces Cleaner production (waste heat recovery + solid waste utilization) integrated Incomplete integration This invention significantly improves In summary, the present invention demonstrates the following advantages in the process of producing rare earth ferrosilicon alloys using the carbothermal method with a DC submerged arc furnace: (1) In terms of efficiency: by improving the raw material grade to over 70% through mineral processing and pre-enrichment, matching the power supply characteristics of the DC furnace with ternary composite carbon, and optimizing the molten pool reduction conditions through layered material distribution, the overall power consumption is reduced by 16.9% compared to the traditional carbothermal method and by 8-13% compared to the existing improved process, and the rare earth yield is stable at over 96%.

[0076] (2) Clean production: The dry absorption defluorination rate of fluorine-containing flue gas is ≥98%, the dust emission concentration is 8~10mg / m³, the comprehensive utilization rate of solid waste is ≥92%, and there is no process wastewater discharge throughout the entire process, which reaches the advanced level of national clean production.

[0077] (3) Product quality: The rare earth content in the alloy fluctuates within ±1.2%, and the content of impurities such as calcium, phosphorus and sulfur is significantly lower than that of traditional processes, meeting the requirements of GB / T4137 standard for high-quality rare earth ferrosilicon alloys.

[0078] (4) Flexibility: Examples 1-3 demonstrate the applicability of the process to different raw material grades (REO 70-77%) and different furnace types (5000-16000kVA). Example 4 verifies the synergistic removal effect of iron additives on impurities, indicating that the process window of the present invention is wide and has strong industrial promotion value.

Claims

1. A process for the efficient and clean production of rare earth ferrosilicon alloys using a carbothermal method in a DC submerged arc furnace, characterized in that, Includes the following steps: (1) Pretreatment of rare earth concentrate: The fluorocarbon cerium rare earth concentrate is pre-enriched to REO content of 70-78% through physical beneficiation, and then subjected to three-stage temperature-controlled roasting. The first stage is from room temperature to 400℃ with a heating rate of 3-5℃ / min; the second stage is 400-550℃ and held for 30-60min; the third stage is 550-750℃ and held for 60-90min, so that the loss on ignition reaches 13-18%. (2) Preparation of composite pellets: The roasted rare earth concentrate and iron additives are mixed in a weight ratio of rare earth metal to iron of 1:0.10 to 0.25, and then mixed with carbonaceous reducing agent and binder to form pellets; the carbonaceous reducing agent is a mixture of coke, charcoal and semi-coke, with a mass percentage of 40 to 60% coke, 20 to 40% charcoal and 10 to 30% semi-coke, and the total amount of carbonaceous reducing agent added is 0.7 to 1.0 times the theoretical amount of carbon required for the complete conversion of rare earth elements into carbides; (3) Ingredient preparation and layered fabric application: The weight ratio of rare earth metals to silicon is controlled at 0.35 to 0.55, and the total amount of carbonaceous reducing agent is 0.88 to 0.96 times the theoretical total carbon content; a layered fabric application method is adopted, with the lower layer, middle layer and upper layer applied in sequence, and the weight ratio of each layer is (15 to 25%): (50 to 70%): (15 to 25%). (4) DC submerged arc furnace smelting: control the furnace temperature at 1950~2150℃, the secondary current density at 15~35A / cm², the electrode insertion depth at 300~600mm, and adopt a continuous feeding method with a feeding rate of 200~400kg / h·MW; (5) Waste heat recovery and tailings treatment: The furnace gas of the electric arc furnace is heated by the waste heat boiler, the slag is treated by magnetic separation and gravity separation to recover iron and rare earth components, the dust is washed by water to recover carbon components, and the valuable components are returned to the corresponding process for reuse.

2. The process according to claim 1, characterized in that, The method of layering the fabric in step (3) is as follows: First layer (lower layer): Mix coarse silica particles of 20-40mm and coarse composite carbonaceous reducing agent particles of 15-30mm in a mass ratio of 2:1 and spread them on the bottom of the furnace, with a thickness of 200-300mm. The second layer (middle layer): rare earth composite pellets, silica with a particle size of 10-25 mm and composite carbonaceous reducing agent with a particle size of 10-20 mm are mixed in a mass ratio of 3:2:1 and added to the mixture, with a thickness of 300-500 mm. The third layer (upper layer): The fine-grained composite carbonaceous reducing agent (5-15 mm) and the fine-grained silica (5-15 mm) are mixed in a 1:1 mass ratio and then covered on the top layer, with a thickness of 100-200 mm.

3. The process according to claim 1, characterized in that, The iron additive is at least one of steel scrap, iron oxide scale, or iron powder, and its chemical composition contains an iron content ≥85wt%.

4. The process according to claim 1, characterized in that, The carbon ratio of the carbonaceous reducing agent mentioned in step (2) is dynamically adjusted according to the REO grade of the rare earth concentrate: when the REO grade is 70-74%, coke is 40-50%, charcoal is 30-40%, and semi-coke is 15-20%; when the REO grade is 74-78%, coke is 50-60%, charcoal is 20-30%, and semi-coke is 15-20%.

5. The process according to claim 1, characterized in that, In step (4), a stepped power supply mode is adopted. The current during the arc ignition stage is 30-40% of the rated value, and it is gradually increased to the rated value. The heating time is 1-2 hours, so that the furnace bottom temperature reaches 1800-1900℃.

6. The process according to claim 1, characterized in that, In step (4), during the smelting process, samples are taken every 30 to 60 minutes to analyze the rare earth content and silicon content in the alloy. When the rare earth content in the alloy is lower than 95% of the target value, silicon powder is added through the furnace top blowing system, and the total carbon content is adjusted to 0.88 to 0.90 times the theoretical amount. When the silicon content in the alloy is higher than 105% of the target value, the total carbon content is adjusted to 0.94 to 0.96 times the theoretical amount.

7. The process according to claim 1, characterized in that, The power factor of the DC submerged arc furnace power supply system is ≥0.92, and the rectification efficiency is ≥0.

96.

8. The process according to claim 1, characterized in that, The waste heat boiler mentioned in step (5) generates steam at 0.5 to 1.5 MPa, part of which is used for the pellet drying process, and part of which is used for power generation or connected to the plant's steam pipeline network.

9. The process according to claim 1, characterized in that, In step (5), after the dust is washed with water to remove soluble salts, the carbonaceous components are returned to the batching system, and the residual tailings are used as building material raw materials.

10. The process according to claim 1, characterized in that, The process has a comprehensive power consumption of 7500-8000 kWh / t, a rare earth yield of ≥96%, a daily output increase of 12-18% per furnace, a dust emission concentration of ≤10 mg / m³, and a solid waste comprehensive utilization rate of ≥90%.

Images